Electric arc furnaces are used for melting scrap and refining iron and steel. These furnaces are typically vessels equipped with a rotatable top with associated electrodes used as the melting source. The furnace is carried by a tiltable base or other movable structure so that, upon completion of melting or refining, molten metal may be poured from a pour spout or tap hole at the lower portion of the vessel into a collecting ladle.
Emissions produced during the operation of an electric arc furnace average one percent to two percent of the weight of the metal poured. The emissions are in the form of gaseous pollutants and fine particulate matter, and are discharged during three distinct stages of furnace operation. The first stage in which waste matter is generated is during the charging of the furnace. During charging, the furnace top and associated electrodes are pivotally moved to one side of the furnace while scrap metal and/or molten metal are added to the furnace vessel. The second stage which generates extensive emissions is the melt-down or refining stage where fumes escape through various openings during melting, during addition of flux or alloying materials and also during temperature determination and sampling. The third stage producing emissions is the tapping stage where molten metal is poured into a receiving or collecting ladle.
Under current environmental controls, emissions generated during operation of the electric arc furnace should not be discharged into the atmosphere. Prior art methods aimed at collecting emissions generated during operation of electric arc furnaces fall into one of four categories.
The first, furnace evacuation, withdraws fumes through an opening in the furnace roof or a side-draft hood around the electrodes. Additional shrouds over the tap hole, electrode holes and charging doors also withdraw furnace fumes from these respective sites. This system does not meet present day codes and, furthermore, does not adequately collect fumes generated when the top is off the furnace for scrap charging nor during the tapping stage.
The second method comprises withdrawing fumes through a canopy in the roof of the building. The canopy is at a distance from the site of fume generation. As a result, abnormally large exhaust air volumes must be moved to attain satisfactory in-plant conditions, and, since the total exhaust is cleaned before it is discharged into the atmosphere, operating costs are high. Therefore, it desirable to keep the the exhausted air volume to a minimum for economic reasons.
The third method, complete shop evacuation, adequately removes waste matter generated during operation of the furnace. However, complete shop evacuation also requires cleaning large volumes of gas. In addition, between evacuations, the shop atmosphere is dirty and hazardous to workers.
The fourth and relatively recent method comprises partially enclosing the furnace within a housing which covers the top and sides of the furnace. The housing is sized to enable tilting of the furnace during the tapping stage where the molten metal is poured into a ladle located outside the enclosure. The housing typically contains a primary exhaust means in the upper area of the housing to collect fumes generated during furnace operations. An additional supplementary exhaust system means is provided adjacent the tapping site to specifically remove pollutants generated during the tapping stage. A major drawback of this method is that the housing does not fully enclose the ladle and thus does not effectively remove fumes generated during tapping. Therefore, some of the extensive fumes generated during the tapping operation inevitably escape into the shop. Also, the primary and secondary exhaust means within such a housing typically operate continuously during all stages of furnace operation and are not selectively controlled to reduce the volume of gas which must be thereafter cleaned before being discharged into the atmosphere.